container-driver: fix volume destination for cache

[thaJeztah]

fixes #749

The container-driver creates a Linux container (as there currently isn't a
Windows version of buildkitd). However, the defaults are platform specific.

Buildx was using the defaults from the buildkit `util/appdefault' package,
which resulted in Buildx running on a Windows client to create a Linux
container that used the Windows location, which causes it to fail:

invalid mount config for type "volume": invalid mount path: 'C:/ProgramData/buildkitd/.buildstate' mount path must be absolute

This patch hard-codes the destination to the default Linux path.

[thaJeztah]

@tonistiigi @crazy-max PTAL

[thaJeztah]

Also wondering if buildx checks if the daemon is running on Linux or Windows (if it's on Windows (native Windows containers) it should not support the container-driver. (or buildkit as a whole, currently)

[zimbres]

Hi, Docker 4.0.0 still happening.

PS D:> docker buildx create --name mybuilder --use
mybuilder

PS D:> docker buildx inspect --bootstrap
[+] Building 2.4s (1/1) FINISHED
=> ERROR [internal] booting buildkit 2.4s
=> => pulling image moby/buildkit:buildx-stable-1 2.4s
=> => creating container buildx_buildkit_mybuilder0 0.0s

[internal] booting buildkit:

Name: mybuilder
Driver: docker-container

Nodes:
Name: mybuilder0
Endpoint: npipe:////./pipe/docker_engine
Error: Error response from daemon: invalid mount config for type "volume": invalid mount path: 'C:/ProgramData/buildkitd/.buildstate' mount path must be absolute

PS D:> docker -v
Docker version 20.10.8, build 3967b7d

[thaJeztah]

@zimbres I don't think buildx 0.6.3 was included in Docker Desktop 4.0.0 yet (it will be included in a future patch release), but you can manually install an updated binary release; https://github.com/docker/buildx#binary-release

[zimbres]

@thaJeztah thank you! Its correct, buildx was 0.6.1. Running fine after override the buildx.